Conventionally, odors have affected human lives in various ways. Many researchers have been interested in, and have studied, why and how odors can be sensed, and have presented results in interpretive articles, reports, etc. on the study of odors. Although some theories about odors have been developed therein, it cannot be said that there is an established theory capable of elucidating the phenomena of odors and being applicable widely to such phenomena.
The adaptation to odors, i.e., the sensing of odors strongly at first but diminishing over time, is one such phenomenon that has heretofore not been fully elucidated. For example, the adaptation to odors such as anethole, ethyl n-butyrate, 2,3-pentanedione, and others was examined in Chemical Senses, Vol. 17(5): 481-491 (1992), but no theory of adaptation to these odors has been set forth.
The adaptation to odors is often inconvenient, if not dangerous, for humans because odors cannot be sensed in spite of the fact that the odors are not dispersed and the odor-emitting substance still remains. Therefore, attempts have been made to prevent the adaptation to odors in order to permit the odors to be sensed for a prolonged period of time. For example, one approach involves the intermittent exposure of odors to test subjects. This approach consists of exposing the subject to an odor-emitting substance for a predetermined period of time, suspending the exposure for a predetermined period of time, and then re-exposing the subject to the odor-emitting substance. However, the controlled release of odors cannot be monitored continuously, so this approach does not provide an effective, nor practical, solution.
Another approach is known in which the subject is exposed to a first odor-emitting substance and then to a second, different odor-emitting substance. In this approach, the adaptation to the first odor is terminated by sensing the second odor. However, the second odor cannot always be sensed, and it is difficult to select the second odor substance because the second odor substance affects the sense of the first odor. Thus, this approach is not satisfactory. Further, the same odor cannot be sensed continuously, so this approach does not provide an effective, nor practical, solution either.